U.S. patent number 7,304,671 [Application Number 10/338,493] was granted by the patent office on 2007-12-04 for imaging apparatus and method for lowering resolution power of an image and modifying the frame period.
This patent grant is currently assigned to Sony Corporation. Invention is credited to Fumihiko Sudo.
United States Patent |
7,304,671 |
Sudo |
December 4, 2007 |
Imaging apparatus and method for lowering resolution power of an
image and modifying the frame period
Abstract
An image pickup device picks up an image. An image signal in
association with the image is filtered in a time axis direction
using a digital low-pass filter to generate an image signal having
a reduced dynamic resolution power, which is equivalent to an image
signal that would be obtained at a longer exposure time. The
filtered signal is then subjected to frame skipping processing to
generate an output image signal having a preferred frame period.
The resultant signal is equivalent to one obtained by a slow
shooting.
Inventors: |
Sudo; Fumihiko (Kanagawa,
JP) |
Assignee: |
Sony Corporation (Tokyo,
JP)
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Family
ID: |
19190790 |
Appl.
No.: |
10/338,493 |
Filed: |
January 8, 2003 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20030128282 A1 |
Jul 10, 2003 |
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Foreign Application Priority Data
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Jan 9, 2002 [JP] |
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P2002-002796 |
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Current U.S.
Class: |
348/241;
348/E5.047; 348/E5.054; 348/E5.078; 348/222.1 |
Current CPC
Class: |
H04N
5/217 (20130101); H04N 5/23241 (20130101); H04N
5/2625 (20130101) |
Current International
Class: |
H04N
5/217 (20060101); H04N 5/228 (20060101) |
Field of
Search: |
;348/241,250,222.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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7 298112 |
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Oct 1995 |
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JP |
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11 32241 |
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Feb 1999 |
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JP |
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11 177930 |
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Feb 1999 |
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JP |
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2001-238189 |
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Aug 2001 |
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JP |
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2001-238190 |
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Aug 2001 |
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JP |
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Primary Examiner: Vu; Ngoc-Yen
Assistant Examiner: Giles; Nicholas G
Attorney, Agent or Firm: Frommer Lawrence & Haug LLP
Frommer; Willaim S. Presson; Thomas F.
Claims
What is claimed is:
1. An apparatus for processing an image signal, comprising: a
filter for digitally filtering an input image signal in a time axis
direction thereof, thereby lowering resolution power for an object
in the time axis direction in association with said image signal;
means for altering a filter coefficient of the filter in accordance
with shutter opening time; and a frame skipping processor for
performing frame skipping processing on the filtered input image
signal, thereby obtaining an output image signal having a preferred
frame period.
2. The apparatus according to claim 1, further comprising a
controller for altering a filtering coefficient of said filter.
3. An apparatus for processing en image signal, comprising: filter
means for digitally filtering an input image signal in a time axis
direction thereof, thereby lowering resolution power for an object
in the time axis direction in association with said image signal;
means far altering a filter coefficient of the filter in accordance
with shutter opening time; and frame-skipping means for performing
frame skipping processing on the filtered input image signal,
thereby obtaining an output image signal having a preferred frame
period.
4. The apparatus according to claim 3, further comprising means for
altering a filtering coefficient of said filter means.
5. A method for processing an image signal, comprising steps of:
digitally filtering an input image signal in a time axis direction
thereof using a digital low-pass filter, thereby lowering
resolution power for an object in the time axis direction in
association with said input image signal; altering a filter
coefficient of a the digital low-pass filter in accordance with
shutter opening time; and frame skipping processing on the filtered
input image signal, thereby obtaining an output image signal having
a preferred frame period.
6. The method for processing an image signal according to claim 5,
further comprising a step of altering a filtering coefficient of
said digital low-pass filter.
7. Imaging equipment, comprising: an image pickup device for
forming an image signal; a digital low-pass filter for digitally
filtering said image signal in a time axis direction thereof,
thereby lowering resolution power for an object in the time axis
direction in association with said image signal; means for altering
a filter coefficient of the digital low pass filter in accordance
with shutter opening time; a frame skipping processor for
performing frame skipping processing on the filtered image signal,
thereby forming an output image signal having a preferred frame
period; and a controller for controlling generation of said image
signal in said image pickup device, filtering characteristic of
said digital low-pass filter, and said frame skipping processing in
said frame skipping processor.
8. The imaging equipment according to claim 7, wherein said
controller allows for varying the frame period of said image signal
generated by said image pickup device within a predetermined range,
and wherein said controller controls frame skipping processing
performed by said frame skipping processor, thereby obtaining an
output image signal having said preferred frame period.
9. The imaging equipment according to claim 7, further comprising:
a switch for selecting one of said image signal generated by said
image pickup device or said image filtered signal by said digital
low-pass filter to output said selected image signal to an image
monitor.
10. The imaging equipment according to claim 7, wherein said
controller varies a filtering coefficient of said digital low-pass
filter, thereby regulating said resolution power.
11. Imaging equipment, comprising: image pickup means for picking
up an object image and forming an image signal; a digital low-pass
filter for digitally filtering said image signal in a time axis
direction thereof, thereby lowering resolution power for an object
in the time axis direction in association with said image signal;
means for altering a filter coefficient of the digital low pass
filter in accordance with shutter opening time; frame-skipping
means for performing frame skipping processing on said filtered
image signal, thereby forming an output image signal having a
preferred frame period; and control means for controlling
generation of said image signal in said image pickup means,
filtering characteristic of said digital low-pass filter, and said
frame skipping processing in said frame skipping means.
12. The imaging equipment according to claim 11, wherein said
control means varies the frame period of said image signal
generated by said image pickup means within a predetermined range,
and wherein said control means controls frame skipping processing
performed by said frame skipping means, thereby obtaining an output
image signal having said preferred frame period.
13. The imaging equipment according to claim 11, further
comprising: switching means for selecting one of said image signal
generated by said image pickup means or said filtered image signal
by said digital low-pass filter to output said selected image
signal to an image monitor.
14. The imaging equipment according to claim 11, wherein said
control means varies a filtering coefficient of said digital
low-pass filter, thereby regulating the lowering in said resolution
power.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an apparatus and a method for processing
an image signal, and imaging equipment.
2. Description of Related Art
In the field of conventional movie production, film cameras have
been used. This type of film camera has a mirror 81 mounted on a
rotary disk shutter 80 off the open area of the shutter, and an
imaging lens 70 and a ground glass 82 of an optical viewfinder
aligned on the optical axis of the mirror 81 as shown in FIG. 1.
When the rotary disk shutter 80 is positioned in the open area,
photographed object images are formed on a film 83, exposing the
film 83. Then this stops displaying the photographed object images
on the optical viewfinder. When the rotary disk shutter 80 is
positioned in the non-open area, frame stepping is carried out for
the film 83, with the photographed object images reflected by the
mirror 81 and displayed on the ground glass 82 of the optical
viewfinder.
Regulation of the period of opening the shutter allows exposure
time to be regulated. Changing the frame-stepping rate allows
special effects to be obtained. For example, by replaying a
high-speed motion picture (i.e. pictures taken at a high
frame-stepping rate) at a normal speed, a high-speed motion of an
object such as a water droplet dropping on the surface of water can
be easily observed in detail. Replaying a low-speed picture
(picture taken at a low frame-stepping rate) at a normal speed can
enhance the reality of, for example, a fighting scene or a
high-speed car chase.
In the field of TV program production, filming (or shooting),
editing, and shipping of programs are in the process of
digitalization. This is also the case in the field of movie
production. That is, owing to the developments of digital
techniques for extended definition pictures and to recent
price-reduction of digital equipment, the digitalization is also
promoted in the world of movie production.
When a video camera utilizing a solid-state image pickup device
such as a charge coupled device (CCD) is used to shoot an object,
control of shutter opening period of a film camera can be emulated
by controlling exposure time using the electronic shutter function
of the solid-state image pickup device. Also by controlling the
read time required for retrieving a signal from the solid-state
image pickup device, film-feeding speed of the film camera can be
emulated. Further, since, unlike an optical viewfinder, an
electronic viewfinder displays an image based on the signal
retrieved from the solid-state image pickup device, the image can
be displayed without any break.
Exposure time of a solid-state image pickup device is controlled
using the electronic shutter function of the device as shown in
FIG. 2A. Thus, a slow shooting (or low-speed shooting) can be
obtained by prolonging the time intervals of images to be obtained,
which can be done by extending the read time required for reading
image data as shown in FIG. 2B. However, if a slow shooting is made
in this way, images displayed on the electronic viewfinder are
refreshed at a longer interval. As result, when panning the camera
to a desired object in motion such that the image of the object is
displayed at the center of the electronic viewfinder according to
the displayed image on the electronic viewfinder if the object is
fast one, the actual position of the image of the object on the
imaged picture differs from the displayed position of the object in
the viewfinder. Thus, the camera cannot correctly follow the
shooting object.
On the other hand, when exposure time is extended while shooting a
moving object, a resolution power for object in a time axis
direction (hereinafter referred to as "dynamic resolution power")
decreases in the shot picture due to an increased movement of the
object during one exposure time, making the displayed image blurred
on the electronic viewfinder.
In a solid-state image pickup device, noise component called dark
current is generated in photo-sensor for performing a photoelectric
conversion. It is known that the dark current increases with
exposure time. Hence, dark current increases when exposure time is
increased to enhance the imaging signal. As a consequence, the
margin of the dynamic range of the photo-sensor decreases with
exposure time, as shown in FIG. 3. For example, when the object is
bright, there are too many of imaging signals for processing to
obtain an image having steady reproduction of halftones so as to
cause a halation on a bright part thereof. If, in an attempt to
increase the margin of the dynamic range, illumination is dimmed or
an amount of light entering the solid-state image pickup device is
reduced by decreasing the aperture of the object lens, a level of
the dark current will not vary but only the intensity of the image
signal decreases, thereby resulting in deterioration of the S/N
ratio. Hence, a noiseless clear image cannot be obtained in this
way.
In a CCD, noise components such as dark current and smear generated
in the transmission lines for reading electric charges generated by
the photoelectric conversion in the photo-sensor, increase with the
read time required for reading signals. Hence, if the read time is
increased, the margin of the dynamic range is further decreased
according to the noise components, resulting in further
deterioration of the S/N ratio.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide an apparatus
and a method for processing an image signal capable of preventing
the lowering of the margin of the dynamic range of the photo-sensor
even in slow shooting and capable of providing a high quality image
without deteriorating its S/N ratio.
It is another object of the invention to provide imaging equipment
capable of preventing the lowering of the margin of the dynamic
range of the photo-sensor even in slow shooting and capable of
providing a high quality image without deteriorating its S/N
ration.
An apparatus for processing an image signal in accordance with one
aspect of the invention comprises a filter for filtering an input
image signal in a time axis direction thereof, thereby lowering the
dynamic resolution power in association with the image signal. The
apparatus also comprises a frame skipping processor for performing
frame skipping processing on the input image signal thus filtered,
thereby obtaining an output image signal having a preferred frame
period. The apparatus further comprises a controller allowing for
altering a filtering coefficient of the filter means.
A method for processing an image signal in accordance with another
aspect of the invention comprises filtering an input image signal
in a time axis direction thereof using a digital low-pass filter,
thereby lowering the dynamic resolution power in association with
the input image signal. The method also comprises performing frame
skipping processing on the input image signal thus filtered,
thereby obtaining an output image signal having a preferred frame
period.
In this arrangement, the input image signal is filtered by means of
a digital low-pass filter in time axis direction, thereby lowering
the dynamic resolution power in association with the input image
signal. The filtering coefficient of the digital low-pass filter is
varied so as to regulate the reduction of the dynamic resolution
power. The image signal having the dynamic resolution power thus
reduced is subjected to the frame skipping processing by the frame
skipping processor to obtain an output image signal having the
reduced dynamic resolution power and a preferred low frame
period.
Imaging equipment in accordance with further aspect of the
invention comprises an image pickup device for forming an image
signal. The imaging equipment also comprises a digital low-pass
filter for filtering the image signal in a time axis direction
thereof, thereby lowering the dynamic resolution power in
association with the image signal. The imaging equipment further
comprises a frame skipping processor for performing frame skipping
processing on the image signal obtained through the filtering,
thereby forming an output image signal having a preferred frame
period. The imaging equipment also comprises a controller for
controlling generation of the image signal in the image pickup
device, filtering characteristic of the digital low-pass filter,
and the frame skipping processing in the frame skipping processor.
The imaging equipment still further comprises a switch for
selecting either the image signal generated by the image pickup
device or the image filtered signal by the digital low-pass filter
to output the selected image signal to an image monitor such as an
image viewfinder.
In this arrangement, the image signal generated by the image pickup
device is filtered by means of a digital low-pass filter in a time
axis direction, thereby lowering the dynamic resolution power in
association with the image signal. The filtering coefficient of the
digital low-pass filter is varied so as to regulate the reduction
of the dynamic resolution power. The image signal having the
dynamic resolution power thus reduced is subjected to the frame
skipping processing to obtain an output image signal having the
reduced dynamic resolution power and a preferred low frame period.
In addition, either the image signal generated by the image pickup
device or the filtered image signal is selected as the output image
signal to be displayed on an image monitor such as an image
viewfinder.
The concluding portion of this specification particularly points
out and directly claims the subject matter of the present
invention. However those skill in the art will best understand both
the organization and method of operation of the invention, together
with further advantages and objects thereof, by reading the
remaining portions of the specification in view of the accompanying
drawing(s) wherein like reference characters refer to like
elements.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram showing operations of a conventional film
camera.
FIGS. 2A and 2B illustrate a case in which time of retrieving
signals is elongated to emulate slow shooting.
FIG. 3 is a diagram showing the margins of the dynamic range of a
photo-sensor for short and long exposure times,
FIG. 4 is a block diagram representation of a video camera in
accordance with the invention.
FIG. 5 shows a block diagram representation of a speed-dependent
processing section.
FIG. 6 is a graph showing step response characteristic of the
digital low-pass filter.
FIG. 7 is a graph showing a frequency characteristic of the digital
low-pass filter.
FIGS. 8A-8D show operations of an image pickup device operating at
a low shooting speed.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
The invention will now be described in detail by way of example
with reference to the accompanying drawings. FIG. 4 shows an
arrangement of a video camera. Incident beam of light enters
through an imaging lens 10 into an imaging pickup section 11 and
then an image of an object is formed on the imaging surface of the
imaging pickup section 11. The image pickup section 11 is formed of
a solid-state image pickup device, e.g. CCD. The image pickup
section 11 retrieves, based on a drive signal RC received from a
drive section 52 as described later, a signal associated with a
picture of the object obtained through photoelectric conversion of
the image of the object. The section 11 generates RGB signals Spa,
for example, on the imaged picture, and supplies the resultant RGB
signals to a pre-processing section 12.
The pre-processing section 12 executes noise-component-removing
processing (e.g. correlated double sampling) on the image signal
Spa to remove therefrom noise components, and supplies the
resultant signal Spa that is removed of noises as an image signal
Spb to an A/D conversion section 13. The A/D conversion section 13
converts the image signal Spb into a digital image signal DVa and
supplies it to a feedback clamp section 14. The feedback clamp
section 14 controls A/D conversion so that an image signal DVa
having a stable black level within a predetermined range can be
obtained with the difference between the black level signal of a
blanking period and a reference signal being detected and supplied
to the A/D conversion section 13. A corrective processing section
15 executes corrections to the image signal Dva such as a shading
correction and compensation for the defects of the image pickup
device. The image signal DVa corrected in the corrective processing
section 15 is supplied as an image signal DVb to a speed-dependent
processing section 20.
FIG. 5 shows a configuration of the speed-dependent processing
section 20. A subtracter 211 and an adder 213 of a digital low-pass
filter 21 and a terminal "a" of a switch 23 receive the image
signal DVb from the corrective processing section 15. The
subtracter 211 also receives an image signal DVf from a one-frame
delay circuit 214, which will be described later, and subtracts the
image signal DVb from the image signal DVf. The resultant
subtraction signal DVc obtained by the subtracter 211 is supplied
to a multiplier 212.
The multiplier 212 receives a filtering coefficient k from a
controller 50 for regulating the filter characteristic of the
digital low-pass filter to obtain the same result as could be
obtained by the regulation of exposure time. The subtraction signal
DVc is multiplied by the filtering coefficient k. The multiplier
212 then obtains and supplies a multiplication signal DVd to the
adder 213.
The adder 213 adds the image signal DVb supplied from the
corrective processing section 15 and the multiplication signal DVd
supplied from the multiplier 212 to generate an image signal DVe.
The image signal DVe thus obtained in the adder 213 is supplied to
the one-frame delay circuit 214, a frame-skipping circuit 22, and
the terminal "b" of the switch 23.
The one-frame delay circuit 214 delays the image signal DVe by 1
frame before it supplies the signal as the image signal DVf to the
subtracter 211. The frame-skipping circuit 22 executes frame
skipping processing based on a skipping control signal CTa received
from the controller 50 to generate an image signal DVg having a
preferred frame number (defined by the ratio of frame period/frame
rate), which signal is supplied to a main line signal processing
section 31 as shown in FIG. 4. The movable terminal "c" of the
switch 23 is connected to a monitoring signal processing section 41
shown in FIG. 4. The switch 23 is adapted to select either the
image signal DVb or the image signal DVe based on a control signal
CTb from the controller 50 and supplies the selected signal as the
image signal DVh to the monitoring signal processing section
41.
The maim line signal processing section 31 executes on the image
signal DVg the signal processing such as gamma (.gamma.) processing
for a video output and outline compensation processing as well as
Knee correction. The image signal DVg done with the above signal
processing is supplied as an image signal DVj to a main line output
processing section 32.
The main line output processing section 32 converts the image
signal DVj to a signal CMout suitable for the device connected to
the video camera before it is output to the device. For example,
when the device complies with component signals and/or composite
signals, the image signal DVj is converted into the signal CMout
suitable for this device. When the image signal is to be
transferred via a serial digital interface that complies with a
format known as SMPTE292M, the output processing section 32 casts
the image signal DVj into the signal CMout suitable for
SMPTE292M-format and outputs it.
The monitoring signal processing section 41 also executes on the
image signal DVh the signal processing such as gamma (.gamma.)
processing and/or outline compensation processing as well as Knee
correction in accordance with an image display device implemented
for monitoring the imaged picture. For example, depending on
whether the image display device is constituted of a CRT or an LCD,
the monitoring signal processing section 41 is switched for any one
of Gamma correction characteristic, Knee correction characteristic,
and outline compensation characteristic. A monitoring output
processing section 42 receives the image signal DVh thus processed
from the monitoring signal processing section 41 as an image signal
DVk.
The monitoring output processing section 42 converts the received
image signal DVk into a signal MTout adequate for a given image
display device of image monitor. For example, if the image display
device is an analog device, the image signal DVk is converted into
an analog signal that is output as the signal MTout.
On the other hand, the controller 50 is connected to an operation
section 51. As a user of a video camera operates the operation
section 51, the section 51 supplies an operation signal PS to the
controller 50. The controller 50 generates various control signals
CT based on the operation signal PS to control the associated
devices so that the video camera can operate as intended by the
user. When, for example, an operation is made on the operation
section 51 to select an imaging speed and/or a shutter time
(shutter opening time), the controller 50 supplies to the
speed-dependent processing section 20 an appropriate filtering
coefficient k in accordance with the selected shutter time, causing
the speed-dependent processing section 20 to generate an image
signal having a dynamic resolution power lowered in accord with the
selected shutter time. The controller 50 generates a control signal
TC for setting a frame period on reading a signal (hereinafter
referred to as "signal read frame period") for the image pickup
section 11 in accord with the selected imaging speed and supplies
the signal to the drive section 52. The controller 50 also
generates control signals CTa and CTb in accord with the selected
imaging speed, which are supplied to the speed-dependent processing
section 20. The drive section 52 generates the drive signal RC
based on the control signal TC and supplies it to the image pickup
section 11 in which a signal is read in a set frame period. The
speed-dependent processing section 20 performs the frame skipping
processing based on the control signal CTa and selects the image
signal based on the control signal CTb. The main line signal output
processing section 32 outputs the image signal corresponding to the
selected imaging speed as well as the monitoring signal output
processing section 42 outputs the image signal in accordance with
user's operation.
Next, operations of the imaging equipment will be described below.
In the speed-dependent processing section 20 as shown in FIG. 5,
the image signal DVb and the image signal DVe obtained by filtering
the image signal DVb in a time axis direction using a digital
low-pass filter 21 have a relationship given by Equation (1) below.
The amplitude characteristic of the digital low-pass filter 21 is
given by Equation (2) below, where a normalization is made such
that the amplitude at .omega.=0 equals 1.
.function..times..function..times..function..times..function..times..time-
s..times..times..times..times..omega..times..times..times..times..times..t-
imes..omega. ##EQU00001## where the angular frequency .omega. is
related to the frame period T of the image signal DVb by
.omega.=.pi./T, k is the filtering coefficient, and n is a positive
integer.
The filter characteristic of the digital low-pass filter 21 of the
speed-dependent processing section 20 varies with k. The step
response of the filter can be obtained for varying k in the range
from 0 to 1. The response becomes less steep for larger k, as shown
in FIG. 6. The frequency characteristic of a digital low-pass
filter 21 is shown in FIG. 7, from which it is seen that the pass
bandwidth decreases with the filtering coefficient k. That is, as
the filtering coefficient k increases, the residual image due to
the image signal DVe increases and high-frequency components are
lost, which manifests that the image is similar to that obtained by
incrementing exposure time to lower the dynamic resolution of the
image. In other words, by filtering the image signal DVb with a
digital low-pass filter 21 having a controlled filtering
characteristic, an image signal of a picture similar to that
obtained by controlling the exposure time (i.e. controlling the
shutter opening time) can be obtained. In short, it is possible to
provide emulated control of shutter opening time by changing the
filtering coefficient k of the digital low-pass filter 21.
To do this, filtering coefficient k of the digital low-pass filter
21 may be altered to obtain an emulated image signal DVe for a slow
shooting by prolonging the exposure time to reduce the dynamic
resolution.
Further, the frame-skipping circuit 22 executes frame skipping
processing on the image signal DVe in accordance with the imaging
speed. In the frame skipping processing performed in the
frame-skipping circuit 22, the number of frames to be skipped may
be regulated to obtain an image signal having a preferred frame
period. Thus, the frame-skipping circuit 22 may emulate variable
frame number function of the film camera.
As a result of the frame skipping by the frame-skipping circuit 22
with a regulated number of skipped frame, it is possible to provide
an image signal DVg of prescribed frame period that results in a
picture similar to the one obtained by reducing the dynamic
resolution with a prolonged exposure time.
It will be understood that if the movable terminal "c" of the
switch 23 is coupled to the terminal "a", the image signal DVb is
supplied to the image display device. Then, an image having a full
dynamic resolution and complete frames can be displayed on the
image display device. This enables slow shooting of an object to
correctly follow the object. Further, by supplying the image signal
DVe to the image display device by coupling the movable terminal
"c" to the terminal "b", an image can be obtained on the image
display device to confirm how much the dynamic resolution is
lowered.
For example, if exposure and reading of the image signal are
carried out by the solid-state image pickup device of the image
pickup section 11 at a frame period of Fa, as shown in FIGS. 8A-8B,
and if three frames per four frames are skipped by the
frame-skipping circuit 22 of the speed-dependent processing section
20, as shown in FIG. 8C, then the image signal DVg (to be supplied
to the signal processing section 31) will have a frame period of
Fa/4 and a reduced dynamic resolution that would be obtained by
prolonging the exposure time in accord with an appropriate
filtering coefficient k.
If the movable terminal "c" of the switch 23 is coupled to the
terminal "a" to select the image signal DVb, then the image display
device of the image monitor will show an image having a frame
period of Fa and a full dynamic resolution, as shown in FIG. 8D.
Moreover, If the movable terminal "c" of the switch 23 is coupled
to the terminal "b" to select the image signal DVe, then an image
having a frame period Fa and based on the image signal DVe supplied
to the main signal processing section 31 can be obtained, from
which one may estimate how much the dynamic resolution thereof is
lowered.
If a slow shooting is not used, the filtering coefficient k is set
to 0, and then no frame skipping will be performed in the
frame-skipping circuit 22. In this instance, the image signal DVb
is the same as the image signal DVe, so that the image signal DVb
is supplied to the signal processing section 31 and the monitoring
signal processing section 41 without performing the frame skipping
processing thereon. Thus, the signal CMout having the maximum frame
rate can be output from the video camera. It is also possible to
display on the image display device an image having full dynamic
resolution. Since the electronic shutter function can be used to
shorten exposure time by controlling the read of the signal from
the solid-state image pickup device, a high-speed shooting is also
possible.
It has been described above that the signal CMout of a video camera
is produced to have a preferred frame rate by altering the number
of skipped frames in the frame skipping circuit 22. However, it is
also possible to generate a signal having a preferred frame rate by
altering not only the number of skipped frames but also the data
read frame period of the image pickup section 11 by the control
signal TC.
In this case, an appropriate number of the frame to be skipped and
an appropriate data read frame period of the image pickup section
11 are selected such that the data read frame period of the image
pickup section 11 does not exceed a predetermined integral multiple
of the minimum frame period that can be attained by the image
pickup device. For example, if the frame period To of a signal
CMout (referred to as "output frame period") does not exceed twice
the minimum frame period Tmin that can be attained by the image
pickup device (i.e. Tmin<To.ltoreq.2Tmin), no frame skipping is
performed and only the signal read frame period Ti is altered to
obtain a signal having a preferred frame rate. If the output frame
period To exceeds 2Tmin but not 3Tmin (i.e.
2Tmin<To.ltoreq.3Tmin), then signal read frame period Ti is
reduced to one half of it, so that frame skipping is performed for
every two frames. When the frame period To exceeds 3Tmin but not
4Tmin (3Tmin<To.ltoreq.4Tmin), the signal read frame period Ti
is reduced to 1/3 of it, so that frame skipping is performed every
three frames. In this manner, when the output frame period To
exceeds n times of the minimum frame period Tmin but not (n+1)
times (nTmin<To.ltoreq.(n+1)Tmin), then the signal read frame
period Ti is reduced to 1/n of it, so that frame skipping is
performed for every n frames. Thus, the signal read frame period Ti
is always shortened.
Since the signal read frame period Ti does not exceed a
predetermined period if the output frame period To is made
arbitrarily longer, the frame period To can be made longer and the
influences of dark current and smear generated in the image pickup
section 11 can be limited. Thus, deterioration of the S/N ratio and
the loss of the margin of dynamic range can be suppressed
accordingly. The number of frames to be skipped can be decreased by
making the signal read frame period longer. In a case where the
movable terminal "c" of the switch 23 is coupled to the terminal
"a" to select the image signal DVb to monitor the imaged picture on
the electronic viewfinder, lowering of the dynamic resolution can
be suppressed since the frame period of the image signal DVb will
remain within the predetermined period.
While the foregoing specification has described preferred
embodiment(s) of the present invention, one skilled in the art may
make many modifications to the preferred embodiment without
departing from the invention in its broader aspects. The appended
claims therefore are intended to cover all such modifications as
fall within the true scope and spirit of the invention.
* * * * *